The present invention relates generally to the field of telecommunications and, more particularly, to delivering services over telephone (switched) networks (POTS, PSTN, ISDN, TCM-ISDN (Time Compression Multiplex-ISDN)) and leased point-to-point two-wire telephone type circuits, such as via modems or other forms of transceivers operating over twisted pair. An exemplary system might have a data signaling rate of 33600 bits/second. For example, multi-channel, multi-carrier communications systems such as discrete multi-tone (DMT) systems may be used and may involve Central Office (CO) exchanges along with Remote Terminal (RT) or Remote Unit (RU) cabinets for delivery to and from Customer Premise Equipment (CP or CPE).
The explosive growth of the internet has created a demand for high data rates for business and residential users (SOHO—small office/home office) that rely on standard analog communications systems, for example plain old telephone systems (POTS) that use a copper wire twisted pair to carry information. The need for high-speed access to the home is increasing due to the availability of information, data, high-bandwidth video and the like from the world wide web. Because of such demand, higher speed modems are required; modems operating at rates of for example 33,600 bits/second are now in use. A multitude of competing communication technologies provide high-speed access to the home such as cable modems and, digital subscriber line (xDSL) equipment. DSL equipment utilize the existing analog POTS that use a copper wire twisted pair to carry the information. Because of bandwidth limitation (4 KHz), and power limitation of the telephone network, line coding schemes are used to encode digital signals into analog signals that convey the analog information over the analog telephone network. Such line coding schemes should avoid the undesirable bandwidth or power increase.
Line coding schemes manipulate the analog carrier signal, which has three attributes, amplitude, phase and frequency. One or more of such attributes may be manipulated by known modulation techniques, one is quadrature amplitude modulation (QAM) whereby the carrier signal's phase and amplitude is modulated to encode more data within a frequency bandwidth. One example of a QAM modulation system sends two bits of information per QAM symbol, where the digital values can be encoded and the corresponding amplitude and phase can be represented using the constellation.
Data from a personal computer or other equipment at the customer premise (CPE) are sent to a transmitter which arranges the data into frame packets; the packetized signal is then quadrature amplitude modulation encoded and error encoded using trellis encoding to improve the noise immunity using a convolutional coder to select a sequence of subsets in a partitioned signal constellation. A numerical symbol vector is trellis encoded. The trellis encoding starts with the most significant symbol and ends with the least significant symbol of the vector, a process which employs convolutional encoding that converts the input symbol to another symbol and then maps the encoded symbol to its corresponding 16 QAM signal constellation point.
VDSL provides symmetric and asymmetric bandwidth and reference standards have been developed by International Telecommunications Union (ITU-T), American National Standards Institute (ANSI) T1/E1, and European Telecommunications Standards Institute (ETSI). Standards of particular interest are T1.424; ITU G.993.1; TS101 270-1; and TS101 270-2, all of which are incorporated herein by reference. Although the invention relates to bi-directional transceivers, it is directed to solving a problem that occurs in only the downstream direction from the exchange/cabinet to the CPE.
A conventional multi-carrier VDSL system normally consists of a data-to-symbol converter, an IFFT, a cyclic extension adder, D/A converter for its transmitter, and an A/D converter, a FFT, a frequency domain equalizer, and a symbol-to-data converter for its receiver.
With reference to an exemplary xDSL deployment, such as that of FIG. 1, a first xDSL system is deployed from the exchange 102, such as a central office, to a first CP 104, while another system is deployed from the cabinet 106, such as a remote terminal, to a second CP 108. In this example, both systems propagate along copper pairs that share a common bundle. The two pairs are electromagnetically coupled with the cabinet system exhibiting an impact into the exchange based system in the form of Far End Cross-Talk (FEXT), represented by arrow 120. It is assumed that both systems share a common bandwidth BE. The distance between the exchange and the cabinet is denoted y, 110. The distance between the cabinet and the customer premise CP1 that receives the downstream exchange payload is x, 112. The distance between the cabinet and its targeted customer premise CP2 may be denoted as z.